RFID has become established in a wide range of applications for the detection and identification of items, allowing substantial amounts of data to be read at greater range than other technologies. Of particular interest is the high frequency (UHF) passive RFID system which promises to offer read ranges of the order of ten meters using tags which do not require their own power source. However these devices typically only have a high probability of being successfully read at distances of a few meters from the reader antennas. This performance typically significantly restricts their applications to environments where local antennas are used for interrogation purposes.
In order for a passive UHF RFID tag to be successfully read, it should receive sufficient radio frequency (RF) power for its internal logic to be activated and transmit back to the reader with sufficient signal-to-noise ratio (SNR). This requirement sets limits on the maximum tag range. However, due to the narrowband nature of the signals, fading effects in real environments generate large variations in the free space loss of both up- and downlink directions and can prevent successful reading of the tag, even well within the maximum read range. Therefore in order to fully deploy these passive UHF RFID tags in real applications, robust reading techniques are required for long range conditions.
Several studies have been undertaken to enhance passive UHF RFID system performance. However, standard RFID systems currently cannot prevent errors (i.e. 100% probability of a successful read). By way of example, “The RF in RFID-passive UHF RFID in practice” by Daniel M. Doubkin proposes a number of ways of improving SNR: The author suggests that inclusion of a 90° phase shift either in in-phase (I) or quadrature (Q) channel in the conventional direct-conversion I/O demodulator improves the SNR of the tag backscattered signal since the phase of the backscattered signal is unpredictable due to its dependent on the distance from the tag.
By way of further example, Mojix (http://www.mojix.com/) has a passive UHF RFID system with phased array of antennas (i.e. the antennas are in the near field region of one another). This allows phased array techniques to be employed, for example digital beam forming to maximise the link budget. This enables improved receiver sensitivity and transmitters which provide radio frequency (RF) signals in the industrial, scientific and medical (ISM) band (902 MHz and 928 MHz) for activating the tags. Details can be found, for example in: WO2007/094868, WO2008/118875 and WO2008/027650. Further background can be found in: EP2146304 and in US 2008/0024273.
The EPC global UHF Class 1 Generation 2 RFID protocol standard allows frequency hopping spread spectrum (FHSS) technique in the US and listen-before-talk technique in the UK to overcome interference in multiple- and dense-interrogator environments
[EPCglobal Specification for RFID Air Interface, online available: http://www.epcglobalinc.org/standards/uhfc1g2/uhfc1g2_1_2_0-standard-20080511.pdf;] [EPCglobal Class Gen 2 RFID Specification, Alien, online available: http://www.rfidproductnews.com/whitepapers/files/AT_wd_EPCGlobal_WEB.pdf].
There is a need for improved techniques for reading in particular UHF passive RFID tags, and for locating such tags, especially in situations where multiple tags may be present in a common region of space.